Optical information storage medium reproduction apparatus and control method of the same

ABSTRACT

In an optical information storage medium reproduction apparatus ( 10 ) for reproducing an optical information storage medium including a plurality of information recording layers each including a recording mark having a length shorter than an optical system resolution limit, reproduction laser power for reading an information recording layer closest to a reproduction-laser-incident surface of the optical information storage medium is set to be lower than reproduction laser power for reading an information recording layer farthest from the reproduction-laser-incident surface but not lower than minimum reproduction laser power that satisfies a reproduction signal characteristic that the optical information storage medium reproduction apparatus ( 10 ) requires. With the arrangement, it is possible to prevent that the information recording layer closest to the reproduction-laser-incident surface is irradiated wrongly with reproduction laser having high reproduction laser power, thereby making it possible to obtain successful reproduction quality. That is, it is possible to realize the optical information storage medium reproduction apparatus ( 10 ) that can set optimum reproduction laser power and perform stable super resolution reproduction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.12/734,169, filed on Apr. 14, 2010 which is a national stage applicationpursuant to 35 U.S.C. §371 of PCT application PCT/JP2008/067327, filedSept. 25, 2008, which claims the benefit of Japanese Application No.273204/2007, filed Oct. 19, 2007. The entire contents of theaforementioned applications are hereby incorporated herein by thisreference.

TECHNICAL FIELD

The present invention relates to an optical information storage mediumreproduction apparatus for reproducing an optical information storagemedium by use of a super resolution reproduction technique, and acontrol method of the optical information storage medium reproductionapparatus.

BACKGROUND ART

In recent years, information technology, information communicationtechnology, and multimedia technology have been greatly advanced. Thisincreases demand for an increase in density and capacity of an opticalinformation storage medium.

To cope with such demand, there have been developed various techniques,such as multilayer techniques for forming information recording layersin a multilayered manner and super resolution techniques using areproduction layer that allows reading of a recording mark shorter thana resolution limit of an optical information storage medium reproductionapparatus.

Initially explained is one of the multilayer techniques. The followingdescribes a double-layer technique, which is most simple among themultilayer techniques. As disclosed in Patent Literature 1, for example,a double-layer optical information storage medium has such a structurethat a first information recording layer and a second informationrecording layer are provided in this order from a side of areproduction-laser-striking surface of the double-layer opticalinformation storage medium, and an intermediate layer is providedtherebetween so that the first and second information recording layersare separated from each other. In this arrangement, since the firstrecording layer is a translucent layer that passes reproduction lighttherethrough, a reproduction laser entering from thereproduction-layer-incident surface can be focused onto either of thefirst and second information recording layers so that information can berecorded in or read from the either of the first and second informationrecording layers. Accordingly, with the use of the double-layertechnique, it is possible to approximately double an informationrecording capacity of the optical information storage medium, by simplearithmetic.

For a structure of an optical information storage medium having at least3 information recording layers, it is also possible to adopt, similarlyto the above example of the double-layer technique, such a structurethat a first recording layer, a second recording layer, a thirdrecording layer, . . . , and an nth recording layer are provided in thisorder from a side of a reproduction-laser-incident surface of theoptical information storage medium, and an intermediate layer issandwiched between two of these information recording layers so as toseparate them from one another.

Further, the above optical information storage media do not employ anysuper resolution techniques. Therefore, in the optical informationstorage media, a shortest recording mark length in each of the layers islonger than a resolution limit of an optical information storage mediumreproduction apparatus.

The following explains about the super resolution techniques. The superresolution technique is a technique for read a signal having a recordingmark length shorter than an optical resolution limit (hereinafter,referred to as optical diffraction limit) in a reproduction opticalsystem. More specifically, a light spot diameter is represented by λ/NAsubstantially, where λ is a wavelength of light emitted from a lightsource and NA is a numerical aperture of an objective lens for forming alight spot.

That is, the super resolution technique is a technique forrecording/reading a recording mark having a recording mark length equalto or less than this optical diffraction limit. Further, reproduction ofan optical information storage medium with the use of the superresolution technique is referred to as super resolution reproduction.

Further, it has been known that a length of a resolution limit of aconventional optical information reproduction apparatus, which does notuse the super resolution technique, is around λ/(4NA), which is onefourth of the light spot diameter. In the following description, thislimit is simply referred to as a resolution limit. It should be notedthat since the resolution limit is actually affected by elements in anoptical system in addition to the theory, there might be somedifferences between an actual value of the resolution limit and atheoretical value obtained from the wavelength and the numericalaperture.

As described above, with the use of the super resolution technique, itis possible to record/read a recording mark having a recording marklength longer than the resolution limit. This can improve an informationrecording capacity of the optical information storage medium.

Patent Literature 2 proposes a super resolution optical informationstorage medium that uses, as the super resolution technique, a superresolution technique utilizing prepits each made up in a form of aconcavity or a convexity, which super resolution technique contributesto reading of information called Super-ROM.

This super resolution optical information storage medium has not beenclarified yet in detail in terms of a reproduction mechanism. However,by using Mo, W, Si, Ge, and the like material instead of Al and Au,which are conventionally used, to form a reflection layer of a read-onlydisk, it is possible to read a signal having a recording pit lengthshorter than the resolution limit, which signal cannot be read by aconventional optical system.

Further, a so-called multilayer super resolution technique, which is atechnique that combines the aforementioned multilayer technique andsuper resolution technique, that is, a technique in which the superresolution technique is applied to each information recording layer in amultilayer optical information storage medium, is expected as atechnique that allows a further increase in the information recordingcapacity. The multilayer super resolution technique may adopt theaforementioned double-layer technique as the multilayer technique. It iscalled a double-layer super resolution technique.

Citation List

Patent Literature 1

Japanese Patent Application Publication, Tokukai, No. 2007-026503 A(Publication Date: Feb. 1, 2007)

Patent Literature 2

Japanese Patent Application Publication, Tokukai, No. 2001-250274 A(Publication Date: Sep. 14, 2001)

Patent Literature 3

Japanese Patent Application Publication, Tokukai, No. 2004-362718 A(Publication Date: Dec. 24, 2004)

Generally, the super resolution reproduction in the super resolutiontechniques requires reproduction laser power higher than power that isnormally required. As one example of the super resolution techniques,there is a mask-type super resolution technique in which a so-calledmask layer is provided on an information recording layer.

There are two types as the mask-type super resolution techniquedepending on characteristics, a heat-mode type and a photon-mode type.In either case, formation of a mask region and an aperture regionrequires a high temperature and a large amount of light in a medium.This requires reproduction laser power higher than power that isnormally required. In addition, it has been also reported that othersuper resolution techniques, such as the super resolution techniquedisclosed in Patent Literature 2, require reproduction laser powerhigher than power that is normally required.

However, reading of an information recording layer with inappropriatelyhigh reproduction laser power may damage recording marks.

SUMMARY OF INVENTION

The present invention is accomplished in view of the above problems. Anobject of the present invention is to provide (i) an optical informationstorage medium reproduction apparatus by which a multilayer superresolution optical information storage medium, which is a high-densityoptical information storage medium, is stably reproduced, and (ii) acontrol method of the optical information storage medium reproductionapparatus.

In order to achieve the above object, an optical information storagemedium reproduction apparatus according to the present invention is anoptical information storage medium reproduction apparatus forreproducing an optical information storage medium in which a pluralityof information recording layers are laminated, each of the plurality ofinformation recording layers including a recording mark having a lengthshorter than an optical system resolution limit, and the opticalinformation storage medium reproduction apparatus reads an informationrecording layer closest to a reproduction-laser-incident surface of theoptical information storage medium, with reproduction laser power thatis set to be lower than reproduction laser power for reading aninformation recording layer farthest from thereproduction-laser-incident surface but not lower than minimumreproduction laser power that satisfies a reproduction signalcharacteristic that the optical information storage medium reproductionapparatus requires.

Further, in order to achieve the above object, an optical informationstorage medium reproduction apparatus according to the present inventionis an optical information storage medium reproduction apparatus forreproducing an optical information storage medium in which a pluralityof information recording layers are laminated, each of the plurality ofinformation recording layers including a recording mark having a lengthnot longer than 120 nm, which optical information storage mediumreproduction apparatus includes an optical system including a laserlight source capable of irradiating laser light having a wavelength ofnot less than 400 nm but not more than 410 nm and an objective lenshaving a numerical aperture of not less than 0.83 but not more than0.87, and the optical information storage medium reproduction apparatusreads an information recording layer closest to areproduction-laser-incident surface of the optical information storagemedium, with reproduction laser power that is set to be lower thanreproduction laser power for reading an information recording layerfarthest from the reproduction-laser-incident surface but not lower thanminimum reproduction laser power that satisfies a reproduction signalcharacteristic that the optical information storage medium reproductionapparatus requires.

Further, in order to achieve the above object, a control methodaccording to the present invention is a control method for controllingan information storage medium reproduction apparatus for reproducing anoptical information storage medium in which a plurality of informationrecording layers are laminated, each of the plurality of informationrecording layers including a recording mark having a length shorter thanan optical system resolution limit, and the control method includessetting, at the time of reading an information recording layer closestto a reproduction-laser-incident surface of the optical informationstorage medium, reproduction laser power to be lower than reproductionlaser power for reading an information recording layer farthest from thereproduction-laser-incident surface but not lower than minimumreproduction laser power that satisfies a reproduction signalcharacteristic that the optical information storage medium reproductionapparatus requires.

In the above arrangement, at the time of reading the informationrecording layer closest to the reproduction-laser-incident surface ofthe optical information storage medium, the optical information storagemedium reproduction apparatus sets reproduction laser power to be lowerthan reproduction laser power for reading the information recordinglayer farthest from the reproduction-laser-incident surface but notlower than minimum reproduction laser power that satisfies areproduction signal characteristic that the optical information storagemedium reproduction apparatus requires. Accordingly, the opticalinformation storage medium reproduction apparatus reads the informationrecording layer closest to the reproduction-laser-incident surface byirradiating the information recoding layer with laser light having thereproduction laser power thus set. As a result, with the abovearrangement, it is possible to prevent that the information recordinglayer closest to the reproduction-laser-incident surface is read withinappropriately high reproduction laser power, thereby making itpossible to prevent a decrease in reflectance and a deterioration inreproduction signal characteristic along with repeat reproduction.Consequently, it is possible to stably reproduce a multilayer superresolution optical information storage medium like the above opticalinformation storage medium.

In a case where a conventional optical information storage mediumreproduction apparatus reproduces a multilayer information storagemedium including n (n is an integer not less than 2) informationrecording layers, the same reproduction laser power is employed to reada first information recording layer, which is first from thereproduction-laser-incident surface, and an nth information recordinglayer, which is nth from the reproduction-laser-incident surface. Forexample, in a case where a conventional double-layer optical informationstorage medium is reproduced, a first information recording layer and asecond information recording layer are read with the same reproductionlaser power.

In view of this, the inventor(s) of the present invention found thefollowing fact. That is, in the case where the first informationrecording layer and the second information recording layer of thedouble-layer super resolution optical information storage medium areread with the same reproduction laser power as such, recording marks ofthe first information recording layer are damaged, thereby resulting inthat information stored in the first information recording layer isrendered unreadable.

The above arrangement of the present invention is applicable to anoptical information storage medium reproduction apparatus including anoptical system constituted by (i) a laser light source capable ofirradiating laser light which is like a blue laser employed by a Blu-rayDisc (registered trademark) reproduction apparatus and which has awavelength of not less than 400 nm but not more than 410 nm, and (ii) anobjective lens having a numerical aperture of 0.85. It should be notedthat even if the numerical aperture has an error of around ±0.02, ithardly affects the advantageous effects of the present invention. Onthis account, the numerical aperture may be not less than 0.83 but notmore than 0.87.

Furthermore, the above arrangement of the present invention is alsoapplicable to reading of information recording layers, such as aninformation recording layer on which a plurality of recording marksincluding a recording mark shorter than 120 nm have been formed, and aninformation recording layer in which information is to be recorded inthe form of a plurality of recording marks including a recording markshorter than 120 nm.

The above optical information storage medium may be a read-only opticalinformation storage medium (read-only type) in which information hasbeen already recorded, or an optical information storage medium(writable/readable type) in which a piece of information has beenrecorded in a part of an information recording layer and another pieceof information may be additionally recorded therein and in which thepiece of information has been recorded in at least one of a plurality ofinformation recording layers.

Additional objects, features, and strengths of the present inventionwill be made clear by the description below. Further, the advantages ofthe present invention will be evident from the following explanation inreference to the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1

FIG. 1 shows one embodiment of the present invention and is a blockdiagram schematically illustrating an arrangement of an essential partof an optical information storage medium reproduction apparatus.

FIG. 2

FIG. 2 shows one embodiment of the present invention and is a blockdiagram schematically showing an arrangement of an optical informationstorage medium reproduction apparatus.

FIG. 3

FIG. 3 is a cross-sectional view schematically illustrating an exemplarydouble-layer optical information storage medium according to the presentinvention.

FIG. 4

FIG. 4 is a graph showing reproduction-laser-power dependence of jitterof an optical information storage medium reproduction apparatus ofReference Example 1 in the present invention.

FIG. 5

FIG. 5 is a graph showing reproduction-laser-power dependence of bERmeasured at the time of reproducing a disk #1 of the present inventionand an optical information storage medium of Comparative Example 1.

FIG. 6

FIG. 6 is a graph showing reproduction-laser-power dependence of CNRmeasured at the time of reproducing a disk #2 of the present inventionand an optical information storage medium of Comparative Example 2.

FIG. 7

FIG. 7 is a graph showing reproduction-time dependence of normalizedreflectance of a first information recording layer of an opticalinformation storage medium as the disk #1 of the present invention.

REFERENCE SIGNS LIST

10 Optical Information Storage Medium Reproduction Apparatus

12 Polarization Beam Splitter (Optical System)

13 Laser Light Resource (Optical System)

15 Detector (Optical System)

19 Control Section

21 Optical Pickup (Optical System)

26 Power Control Section

100 Optical Information Storage Medium

100 a Super Resolution Optical Information Storage Medium

100 d Double-layer Super Resolution Optical Information Storage Medium(Optical Information Storage Medium)

100 e Double-layer Super Resolution Optical Information Storage Medium(Optical Information Storage Medium)

101 Light-transmitting Layer

102 First Information Recording Layer (Information Recording LayerClosest to Reproduction-laser-incident Surface)

103 Intermediate Layer

104 Second Information Recording Layer (Information Recording LayerFarthest from Reproduction-laser-incident Surface)

105 Substrate

Description Of Embodiments

One embodiment is described below with reference to FIG. 1 through FIG.7.

(Optical Information Storage Medium Reproduction Apparatus)

FIG. 2 is a block diagram schematically illustrating one exemplaryarrangement of an optical information storage medium reproductionapparatus according to the present embodiment. An optical informationstorage medium reproduction apparatus 10 illustrated in FIG. 2 canoptically reproduce an optical information storage medium 100. Theoptical information storage medium 100 may be a double-layer superresolution optical information storage medium 100 a or a normal opticalinformation storage medium 100 b other than the double-layer superresolution optical information storage medium.

The present embodiment deals with, as an example, a case where thedouble-layer super resolution optical information storage medium 100 ais reproduced as a multilayer super resolution optical informationstorage medium that is reproduced by the optical information storagemedium reproduction apparatus 10. However, the present embodiment is notlimited to this, and a super resolution optical information storagemedium having at least 3 layers may be reproduced by the opticalinformation storage medium reproduction apparatus 10. Further, thenormal optical information storage medium 100 b that is reproduced bythe optical information storage medium reproduction apparatus 10 may bean optical information storage medium having at least 2 informationrecording layers.

As illustrated in FIG. 2, the optical information storage mediumreproduction apparatus 10 includes a laser control circuit 14A, a signalprocessing circuit 14B, a head amplifier 16, an RF amplifier 17A, asignal processing circuit 17B for RF signals, a servo processing circuit18, a control section 19, a spindle motor 20, an optical pickup 21, andan optical pickup motor 22. The optical pickup 21 includes apolarization beam splitter 12, a laser light source 13, and a detector15.

In the optical information storage medium reproduction apparatus 10,initially, the spindle motor 20 rotates the optical information storagemedium 100, and the control section 19 controls the optical pickup motor22 to move the optical pickup 21. Then, the laser control circuit 14Acauses the laser light source 13 to emit reproduction laser light.

The signal processing circuit 14B is for generating a signal for use inthe laser control circuit 14A. For example, in a case where the opticalinformation storage medium reproduction apparatus 10 has a function ofrecording information in an optical information storage medium, thesignal processing circuit 14B generates a recording signal so that thelaser control circuit 14A controls the laser light source 13 by use ofthe recording signal.

The reproduction laser light passes through the polarization beamsplitter 12 and then strikes the optical information storage medium 100.Reflection light reflected off the optical information storage medium100 returns to the polarization beam splitter 12 and then reaches thedetector 15 via the polarization beam splitter 12. The optical pickup 21includes an objective lens (not shown). A laser wavelength and anumerical aperture for use in a Blue-ray Disc (registered trademark)reproduction apparatus may be used, respectively, as a laser wavelengthof laser light emitted from the laser light source 13 and as a numericalaperture of the objective lens. In this case, the laser wavelength issubstantially 405 nm, and the numerical aperture is substantially 0.85.

The detector 15 outputs an electrical signal based on the receivedreflection light. The electrical signal is fed to the head amplifier 16so as to be amplified. The electrical signal thus amplified is then fedto the servo processing circuit 18 so as to cause the servo processingcircuit 18 to perform various servo controls. Further, the electricalsignal is fed to the RF amplifier 17A so as to be amplified further, andthen fed to the signal processing circuit 17B. The electrical signal isthen fed to the control section 19 via the signal processing circuit17B.

(Control Section of Optical Information Storage Medium ReproductionApparatus)

By referring to FIG. 1, the following describes the control section 19of the optical information storage medium reproduction apparatus 10,more specifically. FIG. 1 is a block diagram illustrating an essentialpart of the control section 19. As illustrated in FIG. 1, the controlsection 19 includes an access position control section 23, a signalprocessing section 24, a medium identifying section 25, a power controlsection 26, and a reproduction clock control section 27.

The access position control section 23 controls the optical pickup motor22 so that the optical pickup 21 can make access to a desired positionon the optical information storage medium 100.

The signal processing section 24 processes a medium identificationsignal fed from the head amplifier 16, and then supplies the mediumidentification signal to the medium identifying section 25. The mediumidentifying section 25 identifies whether or not the optical informationstorage medium 100 is a double-layer super resolution opticalinformation storage medium 100 a, based on the medium identificationsignal supplied from the signal processing section 24.

When the optical information storage medium 100 is identified as thedouble-layer super resolution optical information storage medium 100 a,based on a result of the identification by the medium identifyingsection 25, the power control section 26 controls the laser controlcircuit 14A so as to set reproduction laser power. More specifically,for reading of a first information recording layer, reproduction laserpower Pr1 for the first information recording layer is set. For readingof a second information recording layer, reproduction laser power Pr2for the second information recording layer is set. The reproductionlaser power Pr1 for the first information recording layer and thereproduction laser power Pr2 for the second information recording layerwill be described later more specifically.

The reproduction clock control section 27 switches a reproduction clockfor use in the signal processing circuit 17B, between a reproductionclock suitable for a normal optical information storage medium 100 b anda reproduction clock suitable for a super resolution optical informationstorage medium 100 a, based on the result of the identification of theoptical information storage medium 100 by the medium identifying section25.

The following explains about the super resolution optical informationstorage medium 100 a with reference to FIG. 3. FIG. 3 is across-sectional view schematically illustrating a double-layerinformation storage medium. In this explanation, assume that the superresolution optical information storage medium 100 a is a double-layersuper resolution optical information storage medium, for example. Thesuper resolution optical information storage medium 100 a includes afloodlight layer 101 a, a first information recording layer 102 a, anintermediate layer 103 a, a second information recording layer 104 a,and a substrate 105 a.

The optical information storage medium reproduction apparatus 10 readsinformation stored in the optical information storage medium 100 by useof a reproduction clock received from the reproduction clock controlsection 27 and an electrical signal received from the RF amplifier 17Avia the signal processing circuit 17B.

Further, the following describes which information recording layer is tobe focused, the first information recording layer 102 a or the secondinformation recording layer 104 a, in a case where the super resolutionoptical information storage medium 100 a is reproduced. When a focussearch process is performed by moving the objective lens (not shown) ofthe optical pickup 21 in order from a position away from the superresolution optical information storage medium 100 a toward a focusdirection, a focus error signal for use in the servo processing circuit18 exhibits an S-shaped characteristic every time the focus error signalpasses through each of the information recording layers. At this time,it is counted how many times the S-shaped characteristic is exhibited,and the optical pickup 21 is controlled in accordance with the countednumber so as to select either the first information recording layer 102a or the second information recording layer 104 a, accordingly.

(How to Identify Optical Information Storage Medium)

Next will be explained how to identify whether an optical informationstorage medium 100 to be reproduced by the optical information storagemedium reproduction apparatus 10 is a double-layer super resolutionoptical information storage medium 100 a or not.

One of the methods is such that medium information indicative of whetheror not the optical information storage medium 100 is a double-layersuper resolution optical information storage medium 100 a is added tothe optical information storage medium 100 in a predetermined manner.

More specifically, medium information indicating that an opticalinformation storage medium to be reproduced is the double-layer superresolution optical information storage medium 100 a is stored in thedouble-layer super resolution optical information storage medium 100 ain the form of recording marks having a recording mark length notshorter than a resolution limit of the optical information storagemedium reproduction apparatus 10. The medium information is read withlow reproduction laser power used for the normal optical informationstorage medium 100 b. Herewith, it is possible to judge whether or notthe optical information storage medium 100 is a double-layer superresolution optical information storage medium 100 a, based on the mediuminformation.

With the arrangement, it is possible to prevent such a problem that thefirst information recording layer or the second information recordinglayer is focused with high reproduction laser power accidentally andthereby rendered unreadable irreversibly. Furthermore, the mediuminformation may be recorded in a form of a barcode indicative of themedium information in the vicinity of an internal opening of the opticalinformation storage medium 100.

Alternatively, medium determination means for judging medium informationmay be used before the spindle motor 20 rotates the optical informationstorage medium 100. For example, the medium information is mechanicallyidentified according to a part of the optical information storage medium100, a cutout of a part of a cartridge housing the optical informationstorage medium 100, or the like.

In a case where it is judged, in accordance with the medium informationand any of the above methods, that the optical information storagemedium 100 is the double-layer super resolution optical informationstorage medium 100 a, the optical information storage mediumreproduction apparatus 10 performs the reproduction with reproductionlaser power for the double-layer super resolution optical informationstorage medium 100 a. At this time, the optical information storagemedium reproduction apparatus 10 reads out other information (i.e.,address information and the after-mentioned minimum reproduction laserpower Pr1 min for the first information recording layer) related to thesuper resolution optical information storage medium. The otherinformation related to the super resolution optical information storagemedium can be read even when the other information is recorded in theform of patterns including a pattern having a recording mark length notlonger than the resolution limit of the optical information storagemedium reproduction apparatus 10. This makes it possible to increasedensity of information including the other information related to thesuper resolution optical information storage medium. Moreover, themedium determination means may be able to judge information on theafter-mentioned reproduction laser power Pr1 for the first informationrecording layer and the after-mentioned reproduction laser power Pr2 forthe second information recording layer.

In this case, it is preferable that the playback of the double-layersuper resolution optical information storage medium start from the firstinformation recording layer. This can prevent such a problem that thefirst information recording layer is accidentally read with thereproduction laser power Pr2 for the second information recording layer,which is high reproduction laser power, at the start of the reproductionand thereby the first information recording layer is rendered unreadableirreversibly.

It should be noted that, to start the reproduction in the order from thefirst information recording layer is preferable only in the case of thedouble-layer super resolution optical information storage medium. Thisis not true for an n-layer super resolution optical information storagemedium in which n is at least 3.

EXAMPLES

Optical Information Storage Medium

The following deals with a super resolution optical information storagemedium 100 a and a normal optical information storage medium 100 b,which are reproduced by the optical information storage mediumreproduction apparatus 10 according to the present embodiment.

One example of a conventional double-layer optical information storagemedium is a double-layer optical information storage medium 100 c ofReference Example 1, having a structure illustrated in FIG. 3.

Reference Example 1

The following describes, with reference to FIG. 3, a case where aconventional optical information storage medium reproduction apparatusplays a conventional double-layer optical information storage medium. Asone exemplary case where the optical information storage mediumreproduction apparatus plays such an optical information storage medium,Reference Example 1 deals with a case where the double-layer opticalinformation storage medium 100 c, which is a conventional opticalinformation storage medium including 2 information recording layers, isreproduced. As illustrate in FIG. 3, the double-layer opticalinformation storage medium 100 c includes a light-transmitting layer 101c, a first information recording layer 102 c, an intermediate layer 103c, a second information recording layer 104 c, and a substrate 105,which are laminated in this order from a reproduction-laser-incidentsurface.

The light-transmitting layer 101 c is formed of a transparent resinlayer (film thickness: 75 μm), the first information recording layer 102c is formed of a translucent reflection film (Au, film thickness: 15nm), the intermediate layer 103 c is formed of a transparentultraviolet-curing resin layer (film thickness: 25 μm), the secondinformation recording layer 104 c is formed of a reflection film (Au,film thickness: 50 nm), and the substrate 105 c is formed of a resinsubstrate.

On the intermediate layer 103 c and the substrate 105 c of thedouble-layer optical information storage medium 100 c to be reproducedby the optical information storage medium reproduction apparatus ofReference Example 1, prepits (not shown) are formed each in the form ofa concavity and/or a convexity. By laminating the translucent reflectionfilm and the reflection film on the prepits of the intermediate layer103 c and the substrate 105 c, respectively, the concavities/convexitiesare transferred onto the translucent reflection film and the reflectionfilm. Consequently, the prepits are formed as recording marks on thefirst information recording layer 102 c and the second informationrecording layer 104 c.

That is, the double-layer optical information storage medium 100 c is aso-called read-only optical information storage medium. Further, theprepits, that is, the recording marks, of the double-layer opticalinformation storage medium 100 c of Reference Example 1 are formed in aform of recording patterns based on a 1-7PP modulation method adopted byBlu-ray Disc (registered trademark), in which 2T mark length, which is ashortest recording mark length, is 149 nm. Here, T indicates a timeequivalent to one period of a clock.

Reproduction-laser-power dependence of jitter of each of the firstinformation recording layer 102 c and the second information recordinglayer 104 c in the double-layer optical information storage medium 100 cof Reference Example 1 is measured by use of a disk measurement devicewhich includes a semiconductor laser that irradiates light having awavelength of 405 nm, which is a wavelength of blue laser light, and anoptical system including a numerical aperture NA of 0.85. The result ofthe measurement is shown in FIG. 4.

The jitter is an index indicative of fluctuation in reproduction signalin a time axis direction. As a value of the jitter becomes smaller, thejitter indicates a better reproduction signal characteristic. Generally,the jitter is often used to show a reproduction signal characteristic ofa medium.

Further, a threshold 30 shown by a bold dotted line in FIG. 4 is athreshold of a jitter value at which, in general, the opticalinformation storage medium reproduction apparatus can stably readinformation stored in the optical information storage medium 100, i.e.,a threshold of the jitter necessary for the optical information storagemedium reproduction apparatus to stably reproduce the opticalinformation storage medium 100.

As shown in FIG. 4, respective jitters of the first informationrecording layer 102 c and the second information recording layer 104 cin the double-layer optical information storage medium 100 c are notdependent on reproduction laser power, and are substantially constantand not more than the threshold 30. The reproduction laser power of theoptical information storage medium reproduction apparatus may take anyvalue as long as the reproduction laser power causes the jitter value tobe not more than the threshold 30. On this account, the double-layeroptical information storage medium 100 c exhibiting such a reproductionsignal characteristic shown in Reference Example 1 can select, fromamong a wide range of laser power, respective values for reproductionlaser power for the first information recording layer 102 c andreproduction laser power for the second information recording layer 104c.

(Reproduction Laser Power in Conventional Optical Information StorageMedium Reproduction Apparatus)

In the conventional optical information storage medium reproductionapparatus, reproduction laser power for reading the first informationrecording layer 102 c and reproduction laser power for reading thesecond information recording layer 104 c are set to the same value. Forexample, in FIG. 4, a value of 0.7 mW or the like value can be employedas the reproduction laser power for each of the first informationrecording layer 102 c and the second information recording layer 104 c.

One of the reasons for this is as below. If the reproduction laser powerfor reading the first information recording layer 102 c and thereproduction laser power for reading the second information recordinglayer 104 c are set to values different from each other, control of thereproduction laser power by APC (automatic power control) or the likebecomes complicated, and it becomes difficult for the opticalinformation storage medium reproduction apparatus to read the firstinformation recording layer 102 c and the second information recordinglayer 104 c at the same reproduction circuit.

For this reason, in the conventional optical information storage mediumreproduction apparatus for reproducing the double-layer opticalinformation storage medium 100 c, the reproduction laser power forreading the first information recording layer and the reproduction laserpower for reading the second information recording layer are set to thesame value.

The inventor(s) of the present invention evaluated the reproductionsignal characteristic according to the difference in laser power on thebasis of indices, such as CNR, jitter, and bER, by use of a multilayersuper resolution optical information storage medium, and found thefollowing fact. That is, reading of a first information recording layerof the multilayer super resolution optical information storage mediumwith reproduction laser power equivalent to that for reading a secondinformation recording layer thereof damages the first informationrecording layer, similarly to the conventional and practical opticalinformation storage medium reproduction apparatus for reproducing adouble-layer optical information storage medium.

[Double-Layer Super Resolution Optical Information Storage Medium]

The multilayer super resolution optical information storage medium willbe explained below more specifically, by taking, as one example, a disk#1 and a disk #2, each of which is a double-layer super resolutionoptical information storage medium. Structures of the disks #1 and #2are explained with reference to the structure of FIG. 3, similarly toReference Example 1.

[Disk #1]

A double-layer super resolution optical information storage medium 100 das the disk #1 includes a light-transmitting layer 101 d, a firstinformation recording layer 102 d, an intermediate layer 103 d, a secondinformation recording layer 104 d, and a substrate 105 d, which arelaminated in this order from a reproduction-laser-incident surface.

The light-transmitting layer 101 d is formed of a transparent resinlayer (film thickness: 75 μm), the first information recording layer 102d is formed of a temperature-sensitive layer (zinc oxide, filmthickness: 60 nm) and a light-absorbing layer (Ta, film thickness: 7.5nm), the intermediate layer 103 d is formed of a transparentultraviolet-curing resin layer (film thickness: 25 μm), the secondinformation recording layer 104 d is formed of a temperature-sensitivelayer (zinc oxide, film thickness: 60 nm) and a light-absorbing layer(Ta, film thickness: 15 nm), and the substrate 105 d is formed of aresin substrate. A super resolution technique adopted by thedouble-layer super resolution optical information storage medium 100 dis based on the technique disclosed in Patent Literature 3.

[Disk #2]

A double-layer super resolution optical information storage medium 100 eas the disk #2 includes a light-transmitting layer 101 e, a firstinformation recording layer 102 e, an intermediate layer 103 e, a secondinformation recording layer 104 e, and a substrate 105 e, which arelaminated in this order from a reproduction-laser-incident surface.

The light-transmitting layer 101 e is formed of a transparent resinlayer (film thickness: 75 μm), the first information recording layer 102e is formed of a functional layer (Si, film thickness: 5 nm), theintermediate layer 103 e is formed of a transparent ultraviolet-curingresin layer (film thickness: 25 μm), the second information recordinglayer 104 e is formed of a functional layer (Si, film thickness: 50 nm),and the substrate 105 e is formed of a resin substrate. A superresolution technique adopted by the double-layer super resolutionoptical information storage medium 100 e is the technique disclosed inPatent Literature 2.

Comparative Examples 1 and 2

Optical information storage media each having the following propertieswere reproduced by the optical information storage medium reproductionapparatus 10, for comparison. A result of the reproduction is describedas below. The optical information storage media reproduced here areoptical information storage media that can be reproduced without any ofthe super resolution techniques, and were reproduced just for comparisonto prove that the double-layer super resolution optical informationstorage media 100 d and 100 e have a super resolution characteristic.

The optical information storage media reproduced in Comparative Examples1 and 2 each include a light-transmitting layer, an informationrecording layer, and a substrate, which are laminated in this order froma reproduction-laser-incident surface.

The light-transmitting layer is formed of a transparent resin layer(film thickness: 100 μm), the information recording layer is formed of areflection film (Au, film thickness: 50 nm for Comparative Example 1, 20nm for Comparative Example 2), and the substrate is formed of a resinsubstrate.

On each of the intermediate layer 103 d and the substrate 105 d of thedouble-layer super resolution optical information storage medium 100 das the disk #1, each of the intermediate layer 103 e and the substrate105 e of the double-layer super resolution optical information storagemedium 100 e as the disk #2, and each of the substrates of ComparativeExamples 1 and 2, prepits each made up in the form of a concavity and/ora convexity are formed.

For the disk #1, the temperature-sensitive layer and the light-absorbinglayer are laminated on the prepits, thereby transferring theconcavities/convexities to the light-absorbing layer. Thus, the prepitsare formed on the light-absorbing layer as recording marks.

Similarly, for the disk #2, the functional layer is laminated on theprepits, thereby transferring the concavities/convexities to thefunctional layer. Thus, the prepits are formed on the functional layeras recording marks. Further, for Comparative Examples 1 and 2, thereflection film is laminated on the prepits, thereby transferring theconcavities/convexities on the reflection film. Thus, the prepits areformed on the reflection film as recording marks.

In view of this, the double-layer super resolution optical informationstorage medium 100 d, the optical information storage medium inComparative Example 1, the double-layer super resolution opticalinformation storage medium 100 e, and the optical information storagemedium in Comparative Example 2 are a so-called read-only opticalinformation storage medium.

The prepits, i.e., the recording marks on the double-layer superresolution optical information storage medium 100 d and the opticalinformation storage medium in Comparative Example 1 are formed in arecording pattern based on the 1-7PP modulation method adopted by theBlu-ray Disc (registered trademark). A 2T mark length, which is theshortest recording mark length, is 93 nm. The 1-7PP modulation method iscalled a random pattern method in which a plurality of recording markshaving different lengths are aligned in a signal-reading direction in aregular manner according to a predetermined method.

The prepits, i.e., the recording marks on the double-layer superresolution optical information storage medium 100 e and the opticalinformation storage medium of Comparative Example 2 are formed in asingle-frequency repeating pattern in which a recording mark length is100 nm. The single-frequency repeating pattern method is called amonotone pattern method in which a mark space ratio is 1:1.

(Measurement of Reproduction Signal Characteristic)

A reproduction signal characteristic of each of the double-layer superresolution optical information storage media 100 d and 100 e, and thenormal optical information storage media in Comparative Examples 1 and 2was measured by use of a disk measurement device at the time ofreproducing these media. Results of the measurement are as follows.

(Measurement of Disk #1)

FIG. 5 is a graph showing reproduction-laser-power dependence of bER(bit Error Rate) measured at the time of reading of (i) the firstinformation recording layer 102 d, (ii) the second information recordinglayer 104 d, each in the double-layer super resolution opticalinformation storage medium 100 d as the disk #1, and (iii) theinformation recording layer in the normal optical information storagemedium of Comparative Example 1. The measurement was carried out withthe use of a disk measurement device which includes a semiconductorlaser that irradiates light having a wavelength of 405 nm, which is awavelength of blue laser light, and an optical system having a numericalaperture NA of 0.85.

The optical system is the same as one for use in a Blu-ray Disc(registered trademark) reproduction apparatus. As described above, theresolution limit is represented by λ/(4NA). Accordingly, a resolutionlimit of the disk measurement device is about 120 nm. On this account,the shortest recording mark length, 93 nm, of the double-layer superresolution optical information storage medium 100 d and 3 is not longerthan the resolution limit of the optical system of the disk measurementdevice.

Moreover, the bER indicates a bit error ratio of a reproduction signal.This is one of indices that represent the reproduction signalcharacteristic of a medium. As a value of the bER is smaller, thereproduction signal characteristic becomes better.

When the shortest recording mark length is not longer than theresolution limit, a value of jitter, which is generally used as theindex that represents the reproduction signal characteristic, worsensmarkedly. As a result, the jitter cannot serve as the index. In thiscase, it is difficult to evaluate the reproduction signal characteristicbased on the value of the jitter.

When the jitter takes a moderate value, the bER takes an adequate value.However, in some cases, even if the jitter does not take a moderatevalue, the bER still may take an adequate value. The bER is anevaluation index value eventually necessary for a practical opticalinformation storage medium reproduction apparatus. If the bER isadequate, a system itself works out. For this reason, the bER was alsoused in this measurement as one of the indices that represent thereproduction signal characteristic of a medium.

A threshold 40 shown by a bold dotted line in FIG. 5 is a threshold ofthe bER at which, in general, an optical information storage mediumreproduction apparatus can stably read information recorded in anoptical information storage medium. In other words, the threshold 40 isa threshold of the reproduction signal characteristic that a generaloptical information storage medium reproduction apparatus requires. Asshown in FIG. 5, respective values of the bER that were measured at thetime of the reading of the first information recording layer 102 d andthe second information recording layer 104 d in the double-layer superresolution optical information storage medium 100 d are sufficientlysmaller than the value of the bER that is measured at the time of thereading of the information recording layer in the optical informationstorage medium in

Comparative Example 1

Further, as apparent from FIG. 5, respective ranges of reproductionlaser power that cause the respective values of the bER, measured at thetime of the reading of the first information recording layer 102 d andthe second information recording layer 104 d in the double-layer superresolution optical information storage medium 100 d, to be not more thanthe threshold 40 are different from each other. More specifically, forthe first information recording layer 102 d, the range of reproductionlaser power is from 1.0 mW to 3.7 mW, and for the second informationrecording layer 104 d, the range of reproduction laser power is 2.7 mWto 5.0 mW.

From the measurement results, it is demonstrated that the range ofreproduction laser power of the first information recording layer 102 dis lower than that of the second information recording layer 104 d.Moreover, it is also demonstrated that, on an axis of the reproductionlaser power, the range of reproduction laser power of the firstinformation recording layer 102 d is placed on a side where thereproduction laser power is relatively small, as compared to the secondinformation recording layer 104 d.

In the double-layer super resolution optical information storage medium,the second information recording layer 104 d is read definitely withreproduction light that passes through the first information recordinglayer 102 d. Therefore, an amount of the laser light that reaches thesecond information recording layer 104 d is reduced. For this reason,reproduction laser power necessary for reading the second informationrecording layer 104 d should be higher than that necessary for the firstinformation recording layer 102 d.

Further, similarly to the conventional optical information storagemedium reproduction apparatus, there exists a value (for example, 2.8 mWin FIG. 5) of the reproduction laser power that allows respective valuesof the bER of the first information recording layer 102 d and the secondinformation recording layer 104 d to be not more than the threshold 40.

(Measurement of Disk #2)

FIG. 6 is graphs showing reproduction-laser-power dependence of CNR(Carrier to Noise Ratio) measured at the time of reading (i) the firstinformation recording layer 102 e, (ii) the second information recordinglayer 104 e, each in the double-layer super resolution opticalinformation storage medium 100 e as the disk #2, and (iii) theinformation recording layer in the optical information storage medium ofComparative Example 2. The measurement was carried out with the use ofthe same disk measurement device as the one used for the measurement ofthe double-layer super resolution optical information storage medium 100d, i.e., the disk measurement device which includes a semiconductorlaser that irradiates light having a wavelength of 405 nm, which is awavelength of blue laser light, and an optical system having a numericalaperture NA of 0.85.

As has been already described, the resolution limit is represented byμ/(4NA). Accordingly, a resolution limit of the disk measurement deviceis around 120 nm. On this account, the recording mark length, 100 nm, ofthe double-layer super resolution optical information storage medium 100e and the optical information storage medium of Comparative Example 2 isnot longer than the resolution limit of the disk measurement device.

The CNR is one of evaluation indices for a single-frequency repeatingsignal in a case where the recording marks are formed by the monotonemethod, and is one of the indices representing the reproduction signalcharacteristic of a medium. As a value of the CNR becomes larger, thereproduction signal characteristic becomes better.

As the evaluation index for a single-frequency repeating signal, not thejitter nor the bER, but the CNR is generally used. For this reason, inthis measurement, the CNR was employed as the index representing thereproduction signal characteristic of a medium.

A threshold 50 shown by a bold dotted line in FIG. 6 is a threshold ofthe CNR at which, in general, an optical information storage mediumreproduction apparatus can stably read information stored in an opticalinformation storage medium. That is, the threshold 50 is a threshold ofthe reproduction signal characteristic that a general opticalinformation storage medium reproduction apparatus requires.

As shown in FIG. 6, respective values of the CNR of the firstinformation recording layer 102 e and the second information recordinglayer 104 e in the double-layer super resolution optical informationstorage medium 100 e are sufficiently larger than that of theinformation recording layer of the optical information storage medium ofComparative Example 2.

Further, as apparent from FIG. 6, respective ranges of reproductionlaser power that cause the respective values of the CNR of the firstinformation recording layer 102 e and the second information recordinglayer 104 e in the double-layer super resolution optical informationstorage medium 100 e to be not less than the threshold 50 are differentfrom each other. More specifically, for the first information recordinglayer 102 e, the range of reproduction laser power is 0.6 mW to 1.2 mW,and for the second information recording layer 104 e, the range ofreproduction laser power is not less than 1.1 mW.

From the measurement results, the inventor(s) of the present inventionfound that, in the double-layer super resolution optical informationstorage medium 100 e, the range of reproduction laser power of the firstinformation recording layer 102 e is lower than that of the secondinformation recording layer 104 e. Moreover, it is also found out that,on an axis of the reproduction laser power, the range of reproductionlaser power of the first information recording layer 102 e is placed ona side where the reproduction laser power is relatively small, ascompared to the second information recording layer 104 e.

Further, similarly to the conventional optical information storagemedium reproduction apparatus, there exists a value (for example, 1.1 mWin FIG. 6) of the reproduction laser power that allows respective valuesof the CNR of the first information recording layer 102 e and the secondinformation recording layer 104 e to be not less than the threshold 50.

Further, the inventor(s) of the present invention also found thefollowing fact. That is, similarly to the conventional opticalinformation storage medium reproduction apparatus, if the firstinformation recording layer 102 d and the second information recordinglayer 104 d in the double-layer super resolution optical informationstorage medium 100 d are read with the same reproduction laser power,the first information recording layer 102 d is damaged during thereading, thereby causing such a problem that information stored in thefirst information recording layer 102 e is rendered unreadable. Thisproblem also happens to the double-layer super resolution opticalinformation storage medium 100 e.

For the double-layer super resolution optical information storage medium100 d, the same reproduction laser power means, for example, 3.0 mW, atwhich the respective values of the bER of the first informationrecording layer 102 d and the second information recording layer 104 dduring the reproduction are not more than the threshold. For thedouble-layer super resolution optical information storage medium 100 d,the same reproduction laser power means, for example, 1.1 mW, at whichthe respective values of the CNR of the first information recordinglayer 102 e and the second information recording layer 104 e are notless than the threshold.

(Number of Reproduction Times)

FIG. 7 is a graph showing a correlation between the number ofreproduction times and normalized reflectance at the time when the firstinformation recording layer 102 d of the double-layer super resolutionoptical information storage medium 100 d is read with predeterminedreproduction laser power. FIG. 7 shows reproduction-time dependence ofnormalized reflectance of a maximum return-light amount of areproduction signal, measured at the time when the first informationrecording layer 102 d of the double-layer super resolution opticalinformation storage medium 100 d is read with reproduction laser powerof 1.2 mW, 1.6 mW, 2.0 mW, and 2.8 mW. The normalized reflectanceindicates reflectance obtained by normalizing reflectance at a certainnumber of reproduction times, based on reflectance at the time when thereproduction is performed once. For example, when the number ofreproduction times is 1, the normalized reflectance is 1.0.

As apparent from FIG. 7, as the reproduction laser power increases, anamount of change in the normalized reflectance increases along with anincrease in the number of reproduction times. This means that thecharacteristic becomes worse. For example, in a case where thereproduction laser power is 2.8 mW and the reproduction is performed2000 times, an occurrence ratio of the change in reflectance increasesto at least 50%.

Generally, for the reproduction of an optical information storagemedium, there may be such a case where an optical pickup temporarilystops on a certain track of the optical information storage medium dueto a pause until the next reproduction command is inputted. Therefore,in order that stable reproduction is realized, the optical informationstorage medium should have reproduction resistance that allows theoptical information storage medium to be reproduced at least 2000 times.

Further, a large change in reflectance may cause poor focus.Furthermore, the large change in reflectance may cause reproductionsignal amplitude to exceed its necessary range for reading informationstored in the optical information storage medium, thereby making itdifficult to read the information. In view of this, in order that theoptical information storage medium is stably reproduced, it is necessaryto restrain the occurrence ratio of the change in reflectance to 10% to20%, in general.

In the case where the first information recording layer 102 d of thedouble-layer super resolution optical information storage medium 100 dis read 2000 times with reproduction laser power of 2.8 mW, thenecessary reproduction resistance and the change characteristic inreflectance are not satisfied. For this reason, it is substantiallyimpossible to employ the reproduction laser power of 2.8 mW for thereproduction of the double-layer super resolution optical informationstorage medium 100 d.

Further, it is also demonstrated, in the measurement results, that in acase where the first information recording layer 102 d of thedouble-layer super resolution optical information storage medium 100 dis read with reproduction laser power of not more than 2.0 mW, the bERdoes not exceed its threshold even after the reproduction is performed2000 times. On the other hand, it is demonstrated that in the case wherethe reproduction is performed 2000 times with reproduction laser powerof 2.8 mW, the value of the bER significantly increases and exceeds thethreshold irreversibly. That is, as the reproduction laser powerincreases, the bER becomes worse along with an increase in the number ofreproduction times. From the viewpoint of the bER characteristic, it issubstantially impossible to employ the reproduction laser power of 2.8mW for the reading of the first information recording layer 102 d of thedouble-layer super resolution optical information storage medium 100 d.

Meanwhile, the second information recording layer 104 d is readdefinitely with a reproduction laser that has passed through the firstinformation recording layer 102 d. That is, at the time when anirradiated reproduction laser reaches the second information recordinglayer 104 d, the reproduction laser power of the irradiated reproductionlaser has decreased. On this account, even in a case where thereproduction laser power is set to 2.8 mW, at the time when thereproduction laser reaches the second information recording layer 104 d,the reproduction laser power is less than 2.8 mW. This allows thereflectance and the bER of the second information recording layer 104 dto be stable.

For this reason, unlike the conventional optical information storagemedium reproduction apparatus, it is practically difficult to read, withthe same reproduction power, the first information recording layer 102 dand the second information recording layer 104 d of the double-layersuper resolution optical information storage medium 100 d.

Furthermore, similarly to the double-layer super resolution opticalinformation storage medium 100 d, it is demonstrated that in a casewhere the first information recording layer 102 e of the double-layersuper resolution optical information storage medium 100 e is read withreproduction laser power of 1.1 mW, the reflectance decreases as thenumber of reproduction times increases, thereby decreasing acharacteristic of the first information recording layer 102 e. Thisultimately causes the first information recording layer 102 e to beunreadable.

For this reason, unlike the conventional optical information storagemedium reproduction apparatus, it is practically difficult to read, withthe same reproduction power, the first information recording layer 102 eand the second information recording layer 104 e of the double-layersuper resolution optical information storage medium 100 e as the disk#2.

As described in the above [Background Art], the super resolutiontechniques generally require high reproduction laser power. Further, thesecond information recording layer 104 e is read definitely withreproduction light that has passed the first information recording layer102 e. Therefore, an amount of laser light that actually reaches thesecond information recording layer 104 e has decreased. As a result,conditions of a necessary temperature and light amount necessary forsuper resolution reproduction are not satisfied. That is, the superresolution reproduction of the second information recording layer 104 erequires reproduction laser power higher than power necessary for thefirst information recording layer 102 e.

(Reproduction Laser Power for Reading First Information Recording Layer)

The following describes reproduction laser power at the time when theoptical information storage medium reproduction apparatus 10 reproducesa multilayer super resolution optical information storage medium inwhich n pieces of information recording layers are laminated. Aninformation recording layer that is nth from areproduction-laser-incident surface side of the multilayer superresolution optical information storage medium is referred to as the nthinformation recording layer. The reproduction laser power explainedbelow is reproduction laser power Pr1 for the first informationrecording layer and reproduction laser power Prn for the nth informationrecording layer. That is, the following deals with, as an example, acase where the double-layer super resolution optical information storagemedium 100 d is reproduced. More specifically, the following describesreproduction laser power Pr2 for a second information recording layerwhere n =2.

The reproduction laser power Pr1 for the first information recordinglayer is reproduction laser power that is set when the opticalinformation storage medium reproduction apparatus 10 reads the firstinformation recording layer 102 d. Further, the reproduction laser powerPr2 for the second information recording layer is reproduction laserpower that is set when the optical information storage mediumreproduction apparatus 10 reads the second information recording layer104 d.

Here, minimum reproduction laser power that satisfies a reproductionsignal characteristic that the optical information storage mediumreproduction apparatus 10 requires at the time of the reading of thefirst information recording layer 102 d is referred to as minimumreproduction laser power Pr1 min for the first information recordinglayer. For example, in FIG. 5, reproduction laser power that causes bERmeasured at the time of the reading of the first information recordinglayer 102 d to be not more than the threshold 40 is in a range from 1.0mW to 3.7 mW. That is, the minimum reproduction laser power Pr1 min forthe first information recording layer can be set to 1.0 mW.

The reproduction laser power Pr2 for the second information recordinglayer may be set to any value within a reproduction laser power rangewhich satisfies the reproduction signal characteristic that the opticalinformation storage medium reproduction apparatus 10 requires at thetime of the reading of the second information recording layer 104 d. Forexample, in FIG. 5, reproduction laser power that causes bER measured atthe time of the reading of the second information recording layer 104 dto be not more than the threshold 40 is in a range from 2.7 mW to 5.0mW. The reproduction laser power Pr2 for the second informationrecording layer can be set to any value within the range.

It should be noted that, the lower a value of the reproduction laserpower Pr2 for the second information recording layer is within theaforementioned laser power range, the more the power consumption of thelaser can be restrained. On this account, as shown in FIG. 5, in a caseof a reproduction signal characteristic that is obtained at the timewhen the reproduction laser power that causes the bER measured at thetime of the reading of the second information recording layer 104 d tobe not more than the threshold 40 is within the range of 2.7 mW to 5.0mW, it is preferable to set the reproduction laser power Pr2 for thesecond information recording layer to 2.7 mW.

How to determine the reproduction laser power Pr2 for the secondinformation recording layer is not especially limited in any manner. Forexample, before the reproduction of the double-layer super resolutionoptical information storage medium 100 d starts, the reproduction laserpower Pr2 for the second information recording layer may be set with theuse of means for identifying information related to settings of thereproduction laser power Pr2 for the second information recording layer.The above means identifies the information before the spindle motor 20rotates the optical information storage medium 100 (for example, in sucha manner that a cutout or the like formed in a part of a disk or a partof a cartridge housing the disk, is recognized mechanically).

Further, in a case where the information related to the settings of thereproduction laser power Pr2 for the second information recording layeris recorded in the form of recording marks having a length not shorterthan the resolution limit of the optical information storage mediumreproduction apparatus 10, the information may be read out first withlow reproduction laser power used for the normal double-layer opticalinformation storage medium 100 c or the like, followed by setting thereproduction laser power Pr2 for the second information recording layer.Alternatively, the reproduction laser power Pr2 for the secondinformation recording layer may be set by test reading with the use of atest read region that is formed in advance in the double-layer superresolution optical information storage medium 100 d. That is, thereproduction laser power may be caused to rise and fall so as to measurereproduction laser power dependence of the reproduction signalcharacteristic, and the reproduction laser power Pr2 for the secondinformation recording layer may be set, accordingly, so that its valueis selected from within a range of the reproduction laser power.

Here, the reproduction laser power Pr1 for the first informationrecording layer is not lower than the minimum reproduction laser powerPr1 min for the first information recording layer but lower than thereproduction laser power Pr2 for the second information recording layer.A relationship among (a) the reproduction laser power Pr1 for the firstinformation recording layer, (b) the reproduction laser power Pr2 forthe second information recording layer, and (c) the minimum reproductionlaser power Pr1 min for the first information recording layer satisfiesthe following expression (1):Pr1 min≦Pr1<Pr2  (1)

One of the reasons why the reproduction laser power Pr1 for the firstinformation recording layer should be lower than the reproduction laserpower Pr2 for the second information recording layer is as follows. Asdescribed above in “Reproduction Laser Power in Conventional OpticalInformation Storage Medium Reproduction Apparatus”, in the conventionaloptical information storage medium reproduction apparatus 10, thereproduction laser power Pr1 for the first information recording layerand the reproduction laser power Pr2 for the second informationrecording layer are set to the same value. However, for the reading ofthe first information recording layer 102 d of the double-layer superresolution optical information storage medium 100 d, if the reproductionlaser power Pr1 for the first information recording layer is equal to orhigher than the reproduction laser power Pr2 for the second informationrecording layer, this may cause the first information recording layer102 d to be unreadable. Further, in a case where the reproduction laserpower Pr1 for the first information recording layer is lower than theminimum reproduction laser power Pr1 min for the first informationrecording layer, the reproduction signal characteristic that the opticalinformation storage medium reproduction apparatus 10 requires cannot besatisfied at the time of reading the first information recording layer102 d. In this case, it is difficult to read the first informationrecording layer 102 d. For this reason, it is necessary that thereproduction laser power Pr1 for the first information recording layershould be not lower than the minimum reproduction laser power Pr1 minfor the first information recording layer.

Further, a reproduction laser power range of the reproduction laserpower Pr1 for the first information recording layer further preferablysatisfies the following expression (2):1.05×Pr1 min≦Pr1≦0.95×Pr2  (2)

One of the reasons is as follows. In general, there may occur productionvariability between individual lasers and assembling variability betweenindividual optical systems in the course of producing opticalinformation storage medium reproduction apparatuses, productionvariability between individual optical information storage media,environmental differences at the time of reproduction, and the like. Inview of this, a reproduction laser power margin, which is a reproductionlaser power range for a practical reproduction signal characteristic, isnecessary for a reproduction system. That is, the reproduction laserpower margin indicates an allowable range within which the reproductionlaser power may vary.

Generally, an amount of the reproduction laser power margin becomessmaller, as accuracy of an optical pickup of the optical system of theoptical information storage medium reproduction apparatus becomeshigher, for example. However, since there is variability betweenindividual media, it is necessary that the reproduction laser powermargin have at least 5% latitude. In view of this, the reproductionlaser power Pr1 for the first information recording layer is set suchthat each of an upper limit and a lower limit of the reproduction laserpower Pr1 has a 5% reproduction laser power margin with respect to therange represented by the expression (1) of the reproduction laser powerPr1 for the first information recording layer. That is, the reproductionlaser power Pr1 for the first information recording layer is set so asto satisfy the relationship represented by the expression (2). Thismakes it possible to prevent that the reading of the first informationrecording layer 102 d of the double-layer super resolution opticalinformation storage medium 100 d is affected by various factors, such asproduction variability between individual light sources for use inindividual optical information storage medium reproduction apparatuses10, assembling variability between individual optical systems for use inindividual optical information storage medium reproduction apparatuses10, production variability between individual double-layer superresolution optical information storage media 100 d, and environmentaldifferences at the time of reproduction.

As a result, it is possible that the reproduction signal characteristicnecessary for the optical information storage medium reproductionapparatus 10 is satisfied. Besides, the changes in the reflectance andthe reproduction signal characteristic, which are caused along with anincrease in the number of reproduction times, occur little as comparedto the conventional optical information storage medium reproductionapparatus. Consequently, the first information recording layer 102 d canbe read stably.

How to determine the minimum reproduction laser power Pr1 min for thefirst information recording layer is not limited in any particularmanner, similarly to the reproduction laser power Pr2 for the secondinformation recording layer. The minimum reproduction laser power Pr1min for the first information recording layer may be determined in thesame manner as the reproduction laser power Pr2 for the secondinformation recording layer, as described above. However, in a casewhere the minimum reproduction laser power Pr1 min for the firstinformation recording layer is determined according to the test reading,the minimum reproduction laser power Pr1 min for the first informationrecording layer is set to a certain value by reflecting a result of thetest reading, differently from the reproduction laser power Pr2 for thesecond information recording layer.

How to determine the reproduction laser power Pr1 for the firstinformation recording layer is also not limited in any particularmanner. However, it is necessary that the reproduction laser power Pr1be within the reproduction laser power range represented by theexpression (1), more preferably, the reproduction laser power rangerepresented by the expression (2), each defined by the minimumreproduction laser power Pr1 min for the first information recordinglayer and the reproduction laser power Pr2 for the second informationrecording layer.

(Movement Between Layers)

Next will be explained about the reproduction laser power Pr1 for thefirst information recording layer and the reproduction laser power Prnfor the nth information recording layer at the time when the opticalinformation storage medium reproduction apparatus moves focus(hereinafter also referred to as “focus movement between layers”) fromthe first information recording layer to the nth information recordinglayer and vice versa during reproduction of a multilayer superresolution optical information storage medium. The following deals witha case, as an example, where the double-layer super resolution opticalinformation storage medium 100 d is reproduced, i.e., a case where n=2.

At the time when the focus is moved (focus movement between layers) fromthe first information recording layer 102 d to the second informationrecording layer 104 d or from the second information recording layer 104d to the first information recording layer 102 d, it is preferable thatthe reproduction laser power be lower than the reproduction laser powerPr2 for the second information recording layer.

In a case where the reproduction laser power is not lower than thereproduction laser power Pr2 for the second information recording layerat the time when the focus is moved from the first information recordinglayer 102 d to the second information recording layer 104 d or from thesecond information recording layer 104 d to the first informationrecording layer 102 d (that is, at the time of focus movement betweenlayers), the focus on the first information recording layer 102 d withsuch high reproduction laser power may cause the first informationrecording layer 102 d to be unreadable irreversibly due to the highreproduction laser power. In contrast, in a case where the reproductionlaser power is lower than the reproduction laser power Pr2 for thesecond information recording layer, it is possible to prevent the aboveproblem in which the first information recording layer 102 d is renderedunreadable irreversibly.

Further, it is more preferable that the reproduction laser power used atthe time when the focus is moved from the first information recordinglayer 102 d to the second information recording layer 104 d or from thesecond information recording layer 104 d to the first informationrecording layer 102 d (i.e., at the time of focus movement betweenlayers) be the reproduction laser power Pr1 for the first informationrecording layer. One of the reasons is as follows.

Assume that Prx is reproduction laser power which is lower than thereproduction laser power Pr2 for the second information recording layerbut which is not the reproduction laser power Pr1 for the firstinformation recording layer. Prx is taken as reproduction laser power atthe time when the focus is moved from the first information recordinglayer 102 d to the second information recording layer 104 d or from thesecond information recording layer 104 d to the first informationrecording layer 102 d (at the time of focus movement between layers).

In this case, the reproduction laser power Prx is different from thereproduction laser power Pr1 for the first information recording layerand the reproduction laser power Pr2 for the second informationrecording layer. Therefore, it is necessary that the reproduction laserpower be once set to Prx before it is changed from the reproductionlaser power Pr1 for the first information recording layer to thereproduction laser power Pr2 for the second information recording layerand vice versa. This causes time loss because one focus movement betweenlayers requires two changes in laser power. Further, if Prx is largelydifferent from the reproduction laser power Pr1 for the firstinformation recording layer and the reproduction laser power Pr2 for thesecond information recording layer, this may causes such a problem thatthe focus on each of the information recording layers may become poor.

In view of this, if the reproduction laser power is set to thereproduction laser power Pr1 for the first information recording layerat the time when the focus is moved from the first information recordinglayer 102 d to the second information recording layer 104 d or from thesecond information recording layer 104 d to the first informationrecording layer 102 d (at the time of focus movement between layers), itis possible to prevent time loss and poor focus.

The above description deals with the optical information storage mediumreproduction apparatus 10 that is capable of reproducing thedouble-layer super resolution optical information storage medium 100 das one of examples of the super resolution optical information storagemedium. However, the double-layer super resolution optical informationstorage medium may be an n-layer optical information storage medium(n≧2), the second information recording layer 104 d may be an nthinformation recording layer, and the reproduction laser power Pr2 forthe second information recording layer may be reproduction laser powerPrn for the nth information recording layer. That is, the opticalinformation storage medium reproduction apparatus 10 may be an opticalinformation storage medium reproduction apparatus that is capable ofreproducing a multilayer super resolution optical information storagemedium having n (n is an integer not less than 2) pieces of informationrecording layers.

One of the reasons is that a relationship between the first informationrecording layer and the nth information recording layer in reproductionlaser power and endurance does not change regardless of whether n is 2or not less than 2.

This is apparent from the following fact, for example. That is, the nthinformation recording layer is read definitely with reproduction lightthat has passed through the first information recording layer.Therefore, in order to obtain a temperature and light amount necessaryfor the super resolution reproduction, reproduction laser power higherthan power necessary for the first information recording layer isrequired.

Further, another reason is as follows. That is, since the firstinformation recording layer needs to pass light therethrough to someextent so as to enable the reading of the nth information recordinglayer, it is necessary that a reflection film, a light-absorbing layer,and a functional layer should be formed thin. This causes unstablestates of materials and the like, thereby resulting in that the changein reflectance or the like in the first information recording layerbecomes large along with an increase in the number of reproductiontimes.

(Advantageous Effects of the Present Invention)

In addition to the above description, the present invention can bedescribed as follows. That is, the present invention relates to anoptical information storage medium reproduction apparatus forreproducing an optical information storage medium in which informationis stored with high density.

The present invention relates to an optical information storage mediumreproduction apparatus for recording and reproducing information withthe use of light such as a laser beam or the like, or for reading aread-only optical information storage medium. More specifically, thepresent invention relates to an optical information storage mediumreproduction apparatus for reproducing an optical information storagemedium, a method for reproducing an optical information storage mediumand an optical information storage medium, each of which employs a superresolution optical information storage medium technique for reading arecording mark that is not longer than an optical resolution limitdefined by a light spot diameter at diffraction limit.

An optical information storage medium reproduction apparatus of thepresent invention is an optical information storage medium reproductionapparatus that is capable of reproducing a multilayer opticalinformation storage medium having n (n is an integer not less than 2)pieces of information recording layers including a reproduction filmthat allows reading of a recording mark shorter than a resolution limitof the optical information storage medium reproduction apparatus. Theoptical information storage medium reproduction apparatus satisfies Pr1min≦Pr1<Prn, where Pr1 is reproduction laser power for reading a firstinformation recording layer, which is closest to areproduction-light-incident surface of the optical information storagemedium, Prn is reproduction laser power for reading an nth informationrecording layer, which is farthest from the reproduction-light-incidentsurface, and Pr1 min is minimum reproduction laser power that satisfiesa reproduction signal characteristic necessary for the opticalinformation storage medium reproduction apparatus at the time of readingthe first information recording layer.

With the arrangement, at the time of reading the first informationrecording layer, which is closest to the reproduction-light-incidentsurface of the multilayer super resolution optical information storagemedium, the reproduction signal characteristic necessary for the opticalinformation storage medium reproduction apparatus is satisfied, therebypreventing a decrease in reflectance and a deterioration in reproductionsignal characteristic along with an increase in the number ofreproduction times, as compared with a conventional optical informationstorage medium reproduction apparatus. As a result, it is possible tostably read the first information recording layer.

An optical information storage medium reproduction apparatus of thepresent invention is an optical information storage medium reproductionapparatus (i) which is capable of reproducing a multilayer opticalinformation storage medium having n (n is an integer not less than 2)pieces of information recording layers each having a recording markshorter than 120 nm and (ii) which includes an optical system includinga laser that irradiates light having a wavelength of about 405 nm and anobjective lens having a numerical aperture of substantially 0.85. Theoptical information storage medium reproduction apparatus satisfies Pr1min≦Pr1<Prn, where Pr1 is reproduction laser power for reading a firstinformation recording layer, which is closest to areproduction-light-incident surface of the optical information storagemedium, Prn is reproduction laser power for reading an nth informationrecording layer, which is farthest from the reproduction-light-incidentsurface, and Pr1 min is minimum reproduction laser power that satisfiesa reproduction signal characteristic necessary for the opticalinformation storage medium reproduction apparatus at the time of readingthe first information recording layer.

In this arrangement, the optical information storage medium reproductionapparatus of the present invention includes an optical system includinga laser that irradiates light having a blue laser wavelength like aBlu-ray Disc (registered trademark) reproduction apparatus and anobjective lens having a numerical aperture of substantially 0.85.Further, in the arrangement, the optical information storage mediumreproduction apparatus satisfies its necessary reproduction signalcharacteristic at the time of reading a first information recordinglayer, which is closest to a reproduction-light-incident surface of aread-only type multilayer super resolution optical information storagemedium. Consequently, with the arrangement, it is possible to prevent adecrease in reflectance and a deterioration in reproduction signalcharacteristic along with an increase in the number of reproductiontimes, as compared to a conventional optical information storage mediumreproduction apparatus, thereby allowing stable reading of the firstinformation recording layer.

An optical information storage medium reproduction apparatus of thepresent invention is an optical information storage medium reproductionapparatus (i) which includes an optical system including a laser thatirradiates light having a wavelength of substantially 405 nm and anobjective lens having a numerical aperture of substantially 0.85 and(ii) which is capable of reproducing a multilayer optical informationstorage medium having n (n is an integer not less than 2) pieces ofinformation recording layers in which information is stored in the formof a plurality of recording marks including a recording mark shorterthan 120 nm. The optical information storage medium reproductionapparatus satisfies Pr1 min≦Pr1<Prn, where Pr1 is reproduction laserpower for reading a first information recording layer, which is closestto a reproduction-light-incident surface of the optical informationstorage medium, Prn is reproduction laser power for reading an nthinformation recording layer, which is farthest from thereproduction-light-incident surface, and Pr1 min is minimum reproductionlaser power that satisfies a reproduction signal characteristicnecessary for the optical information storage medium reproductionapparatus at the time of reading the first information recording layer.

In this arrangement, the optical information storage medium reproductionapparatus of the present invention includes an optical system includinga laser that irradiates light having a blue laser wavelength like aBlu-ray Disc (registered trademark) reproduction apparatus and anobjective lens having a numerical aperture of substantially 0.85.Further, the optical information storage medium reproduction apparatussatisfies its necessary reproduction signal characteristic at the timeof reading a first information recording layer, which is closest to areproduction-light-incident surface of a writable type multilayer superresolution optical information storage medium. Consequently, with thearrangement, it is possible to prevent a decrease in reflectance and adeterioration in reproduction signal characteristic along with anincrease in the number of reproduction times, as compared to aconventional optical information storage medium reproduction apparatus,thereby allowing stable reading of the first information recordinglayer.

The optical information storage medium reproduction apparatus of thepresent invention satisfies 1.05×Pr1 min≦Pr1≦0.95×Prn.

Here, the optical information storage medium reproduction apparatusrequires a margin for reproduction power for its reproduction system soas to cope with environmental differences at the time of reproduction.The environmental differences at the time of reproduction may encompassproduction variability between individual lasers for use in individualoptical information storage medium reproduction apparatuses, assemblingvariability between individual optical systems for use in individualoptical information storage medium reproduction apparatuses, productionvariability between individual optical information storage media, andthe like.

Further, as the optical information storage medium reproductionapparatus is more accurate, a necessary amount of the margin becomessmaller. However, a general optical information storage mediumreproduction apparatus requires a margin ratio of at least 5%. On thisaccount, in the present invention, Pr1 is set such that each of an upperlimit and a lower limit of Pr1 has a 5% reproduction power margin. Thatis, the optical information storage medium reproduction apparatus of thepresent invention satisfies 1.05×Pr1 min≦Pr1≦0.95×Prn.

With the arrangement, the optical information storage mediumreproduction apparatus of the present invention satisfies its necessaryreproduction signal characteristic at the time of reading a firstinformation recording layer, which is closest to areproduction-light-incident surface of a multilayer super resolutionoptical information storage medium, without being affected by productionvariability between individual lasers for use in individual opticalinformation storage medium reproduction apparatuses, assemblingvariability between individual optical systems for use in individualoptical information storage medium reproduction apparatuses, productionvariability between individual optical information storage media, andenvironmental differences at the time of reproduction. As a result, itis possible to restrain a decrease in reflectance and a deterioration inreproduction signal characteristic along with an increase in the numberof reproduction times, as compared to a conventional optical informationstorage medium reproduction apparatus, thereby making it possible tomore stably read the first information recording layer.

The optical information storage medium reproduction apparatus of thepresent invention is arranged such that reproduction laser power is setlower than Prn at the time of moving its focus (hereinafter justreferred to as “movement between layers) from a first informationrecording layer onto an nth information recording layer or from the nthinformation recording layer onto the first information recording layer.

In a case where the reproduction laser power is set not lower than Prnat the time of moving the focus from the nth information recording layeronto the first information recording layer, there may occur such aproblem that the first information recording layer may be renderedunreadable irreversibly. In contrast, if the reproduction laser power isset lower than Prn at this time, it is possible to prevent the problemthat the first information recording layer is rendered unreadableirreversibly. The reproduction laser power is preferably set to Pr1.

Assume Prx is reproduction laser power which is lower than Prn but whichis not Pr1 and the reproduction laser power is set to Prx at the time ofmoving the focus from the first information recording layer to the nthinformation recording layer or from the nth information recording layerto the first information recording layer. At this time, the reproductionlaser power is set to a value that is different from both reproductionlaser power Pr1 for the first information recording layer andreproduction laser power Prn for the nth information recording layer.Therefore, the reproduction laser power should be once set to Prx beforethe reproduction laser power is changed from Pr1 to Prn and vice versa.That is, the laser power should be changed in two stages for one focusmovement between layers, thereby causing the focus movement betweenlayers to take time. Further, if Prx is set to a value largely differentfrom Pr1 or Prn, there may occur such a problem that the focus on eachof the information recording layers may become poor.

For these reasons, when the reproduction laser power is set to Pr1 atthe time of the focus movement between layers, it is possible to focusan intended information recording layer in a time efficient manner.

The optical information storage medium reproduction apparatus of thepresent invention performs reproduction in the order from a firstinformation recording layer in a case of an optical information storagemedium where n=2. In a case of an optical information storage mediumreproduction apparatus that reads a second information recording layerfirst, there may occur such a problem that the first informationrecording layer is focused accidentally so that the first informationrecording layer is read with Pr2, which is high reproduction laserpower, thereby causing the first information recording layer to beunreadable irresistibly. In contrast, with the above arrangement, sincethe optical information storage medium reproduction apparatus of thepresent invention reads the first information recording layer first, itis possible to prevent such a problem. Further, in a case where adouble-layer super resolution optical information storage medium, wheren=2, is a medium from which the optical information storage mediumreproduction apparatus can identify values of Pr1 and Pr2 before itstarts to reproduce the medium, the optical information storage mediumreproduction apparatus starts to read the first information recordinglayer with Pr1 and the second information recording layer with Pr2. Thisyields an effect that the reproduction can be performed in a timeefficient manner.

The present embodiment discloses an optical information storage mediumreproduction apparatus for reproducing an optical information storagemedium. However, the present invention is not limited to this, and mayinclude a read-only or write-only apparatus and a read/write apparatus.Further, the usage of such an apparatus is not limited in any particularmanner, and the apparatus may be stationary, portable, or the like.

As described above, an optical information storage medium reproductionapparatus, according to the present invention, for reproducing anoptical information storage medium in which a plurality of informationrecording layers are laminated, each of the plurality of informationrecording layers including a recording mark having a length shorter thanan optical system resolution limit. The optical information storagemedium reproduction apparatus of the present invention reads aninformation recording layer closest to a reproduction-laser-incidentsurface of the optical information storage medium, with reproductionlaser power that is set to be lower than reproduction laser power forreading an information recording layer farthest from thereproduction-laser-incident surface but not lower than minimumreproduction laser power that satisfies a reproduction signalcharacteristic necessary for the optical information storage mediumreproduction apparatus.

Further, an optical information storage medium reproduction apparatusaccording to the present invention, is an optical information storagemedium for reproducing an optical information storage medium in which aplurality of information recording layers are laminated, each of theplurality of information recording layers including a recording markhaving a length not longer than 120 nm, the optical information storagemedium reproduction apparatus includes an optical system including alaser light source capable of irradiating laser light having awavelength of not less than 400 nm but not more than 410 nm and anobjective lens having a numerical aperture of not less than 0.83 but notmore than 0.87. The optical information storage medium reproductionapparatus of the present invention reads an information recording layerclosest to a reproduction-laser-incident surface of the opticalinformation storage medium, with reproduction laser power that is set tobe lower than reproduction laser power for reading an informationrecording layer farthest from the reproduction-laser-incident surfacebut not lower than minimum reproduction laser power that satisfies areproduction signal characteristic that the optical information storagemedium reproduction apparatus requires.

Furthermore, an optical information storage medium reproductionapparatus according to the present invention is an optical informationstorage medium reproduction apparatus for reproducing an opticalinformation storage medium in which a plurality of information recordinglayers are laminated, each of the plurality of information recordinglayers including a recording mark having a length not longer than 120nm, the optical information storage medium reproduction apparatusincludes an optical system including a laser light source capable ofirradiating laser light having a wavelength of not less than 400 nm butnot more than 410 nm and an objective lens having a numerical apertureof 0.85. The optical information storage medium reproduction apparatusof the present invention reads an information recording layer closest toa reproduction-laser-incident surface of the optical information storagemedium, with reproduction laser power that is set to be lower thanreproduction laser power for reading an information recording layerfarthest from the reproduction-laser-incident surface but not lower thanminimum reproduction laser power that satisfies a reproduction signalcharacteristic that the optical information storage medium reproductionapparatus requires.

A control method, according to the present invention is a method forcontrolling an information storage medium reproduction apparatus forreading an optical information storage medium in which a plurality ofinformation recording layers are laminated, each of the plurality ofinformation recording layers including a recording mark having a lengthshorter than an optical system resolution limit. The control method ofthe present invention includes setting, at the time of reading aninformation recording layer closest to a reproduction-laser-incidentsurface of the optical information storage medium, reproduction laserpower to be lower than reproduction laser power for reading aninformation recording layer farthest from thereproduction-laser-incident surface but not lower than minimumreproduction laser power that satisfies a reproduction signalcharacteristic that the optical information storage medium reproductionapparatus requires.

In the above arrangement, at the time of reading the informationrecording layer closest to the reproduction-laser-incident surface ofthe optical information storage medium, the optical information storagemedium reproduction apparatus sets reproduction laser power to be lowerthan reproduction laser power for reading the information recordinglayer farthest from the reproduction-laser-incident surface but notlower than minimum reproduction laser power that satisfies areproduction signal characteristic that the optical information storagemedium reproduction apparatus requires. Accordingly, the opticalinformation storage medium reproduction apparatus reads the informationrecording layer closest to the reproduction-laser-incident surface byirradiating the information recoding layer with laser light having thereproduction laser power thus set. As a result, with the abovearrangement, it is possible to prevent that the information recordinglayer closest to the reproduction-laser-incident surface is read withinappropriately high reproduction laser power, thereby making itpossible to prevent a decrease in reflectance and a deterioration inreproduction signal characteristic along with repeat reproduction.Consequently, it is possible to stably reproduce a multilayer superresolution optical information storage medium like the above opticalinformation storage medium.

In a case where a conventional optical information storage mediumreproduction apparatus reproduces a multilayer information storagemedium including n (n is an integer not less than 2) informationrecording layers, the same reproduction laser power is employed to reada first information recording layer, which is first from thereproduction-laser-incident surface, and an nth information recordinglayer, which is nth from the reproduction-laser-incident surface. Forexample, in a case where a conventional double-layer optical informationstorage medium is reproduced, the first information recording layer andthe second information recording layer are read with the samereproduction laser power.

In view of this, the inventor(s) of the present invention found thefollowing fact. That is, in the case where the first informationrecording layer and the second information recording layer of thedouble-layer super resolution optical information storage medium areread with the same reproduction laser power as such, recording marks ofthe first information recording layer are damaged, thereby resulting inthat information stored in the first information recording layer isrendered unreadable.

The above arrangement of the present invention is applicable to anoptical information storage medium reproduction apparatus including anoptical system constituted by (i) a laser light source capable ofirradiating laser light which is like a blue laser employed by a Blu-rayDisc (registered trademark) reproduction apparatus and which has awavelength of not less than 400 nm but not more than 410 nm, and (ii) anobjective lens having a numerical aperture of 0.85. It should be notedthat even if the numerical aperture has an error of around ±0.02, ithardly affects the advantageous effects of the present invention. Onthis account, the numerical aperture may be not less than 0.83 but notmore than 0.87.

Furthermore, the above arrangement of the present invention is alsoapplicable to reading of information recording layers, such as aninformation recording layer on which a plurality of recording marksincluding a recording mark shorter than 120 nm have been formed, and aninformation recording layer in which information is to be recorded inthe form of a plurality of recording marks including a recording markshorter than 120 nm.

The above optical information storage medium may be a read-only opticalinformation storage medium (read-only type) in which information hasbeen already recorded, or an optical information storage medium(writable/readable type) in which a piece of information has beenpartially recorded and another piece of the information may beadditionally recorded and in which the piece of information has beenrecorded in at least one of a plurality of information recording layers.

In addition, the optical information recording medium reproductionapparatus according to the present invention is arranged such that thereproduction laser power with which the optical information recordingmedium reproduction apparatus reads the information recording layerclosest to the reproduction-laser-incident surface is set to be nothigher than 0.95 times the reproduction laser power for reading theinformation recording layer farthest from the reproduction-laserincident surface, but not lower than 1.05 times the minimum reproductionlaser power that satisfies the reproduction signal characteristic thatthe optical information recording layer requires at the time of readingthe information recording layer closest to thereproduction-laser-incident surface.

Generally, there are various factors that affect quality in reproductionat the time of reproduction. Examples of such factors are productionvariability between individual lasers for use in individual opticalinformation storage medium reproduction apparatuses, assemblingvariability between individual optical systems for use in individualoptical information storage medium reproduction apparatuses, productionvariability between individual optical information storage media, andenvironmental differences at the time of reproduction.

In order to cope with such factors, a reproduction laser power margin,which is a reproduction laser power range for a practical reproductionsignal characteristic, is required. More specifically, the reproductionlaser power margin indicates an allowable range within which thereproduction laser power may vary. Generally, as accuracy of, forexample, an optical pickup of the optical system of the opticalinformation storage medium reproduction apparatus becomes higher, anamount of the reproduction laser power margin becomes smaller. However,since there is variability between individual media and the like, it isnecessary that the reproduction laser power have at least 5% latitudewith respect to each of theoretical upper limit and lower limit.

As such, in the above arrangement, the reproduction laser power marginhas at least 5% latitude with respect to each of the theoretical upperlimit and lower limit. As a result, the arrangement further makes itpossible that the reproduction can be performed without being affectedby production variability between individual laser light sources for usein individual optical information storage medium reproductionapparatuses, assembling variability between individual optical systemsfor use in individual optical information storage medium reproductionapparatuses, production variability between individual opticalinformation storage media, environmental differences at the time ofreproduction, and the like.

Further, the optical information storage medium reproduction apparatusof the present invention is arranged such that at the time of focusmovement from an information recording layer onto another informationrecording layer, the optical information storage medium reproductionapparatus moves the focus from the information recording layer to theanother information recording layer with reproduction laser power beingmaintained to be lower than the reproduction laser power for reading theinformation recording layer farthest from thereproduction-laser-incident surface.

With the arrangement, even when the focus is moved from the informationrecording layer farthest from the reproduction-laser-incident surfaceonto the information recording layer closest to thereproduction-laser-incident surface and vice versa, it is possible torestrain that the information recording layer closest to thereproduction-laser-incident surface is read with inappropriately highreproduction laser power. This results in that it is possible to preventa decrease in reflectance and a deterioration in reproduction signalcharacteristic along with repeat reproduction.

In the above arrangement, it is preferable that the optical informationstorage medium reproduction apparatus move the focus from theinformation recording layer to the another information recording layerwith reproduction laser power for reading one of the informationrecording layers that is closer to the reproduction-laser-incidentsurface.

In the arrangement, when the focus is moved between layers, thereproduction laser power is changed only once. This can improveefficiency and further prevent poor focus.

Further, the optical information storage medium reproduction apparatusaccording to the present invention is arranged such that in a case wherethe optical information storage medium includes 2 information recordinglayers, the optical information storage medium reproduction apparatusreproduces the optical information storage medium in the order from oneof the 2 information recording layers that is closer to thereproduction-laser-incident surface than the other one of the 2information recording layers.

Assume that an optical information storage reproduction apparatusreproduces a double-layer optical information storage medium, whichincludes 2 information recording layers, and the optical informationstorage medium reproduction apparatus performs reproduction in the orderfrom one of the information recording layers that is farther from areproduction-laser-incident surface of the medium than the other one. Inthis case, if the other one of the information recording layers that iscloser to the reproduction-laser-incident surface is read wrongly withreproduction laser power for the information recording layer fartherfrom the reproduction-laser-incident surface, there may occur such aproblem that the information recording layer closer to thereproduction-laser-incident surface is rendered unreadable irreversibly.

However, with the above arrangement of the present invention, it ispossible to prevent that the information recording layer closer to thereproduction-laser-incident surface is irradiated with light having suchinappropriately high reproduction laser power, thereby preventing adeterioration in reproduction characteristic of the informationrecording layer closer to the reproduction-laser-incident surface.

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

Industrial Applicability

The present invention can be widely and preferably applied to an opticalinformation storage medium reproduction apparatus and the like whichperforms super resolution reproduction of a multilayer opticalinformation storage medium.

The invention claimed is:
 1. A reproduction method for reproducing arewritable and readable optical information storage medium (i) whichincludes n (n is an integer of not less than 2) pieces of informationrecording layers each including a recording mark having a length shorterthan an optical system resolution limit, the recording mark being formedbased on a 1-7PP modulation method, and (ii) which includes an arealocated near a center hole of the rewritable and readable opticalinformation storage medium in which area medium information on astructure of the rewritable and readable optical information storagemedium is stored, said reproduction method comprising: settingreproduction laser power to satisfy Pr1<Pm and Pr1 of not less than 1.0mW but less than 1.1 mW, where Pr1 is reproduction laser power forreading a first information recording layer, in the rewritable andreadable optical information storage medium, that is closest to areproduction-laser-incident surface of the rewritable and readableoptical information storage medium, and Prn is reproduction laser powerfor reading an nth information recording layer, in the rewritable andreadable optical information storage medium, that is farthest from thereproduction-laser-incident surface; and reading recorded informationfrom the rewritable and readable optical information storage medium byuse of a reproduction clock determined based on the medium information.